The present invention relates to a catalyst composition for the combustion of particulates from diesel engine exhaust, and more particularly, to a catalyst composition which facilitates the oxidation of soot during the regeneration of diesel particulate filters and which contains little or no platinum group metals.
In recent years, environmental regulations in the United States and Europe restricting diesel particulate emissions have necessitated improvements in the removal of particulates from diesel engine emissions. Such particulates generally consist of carbonaceous particulates in the form of soot. Currently, the most commonly used method for removing soot from engine exhaust is the use of a diesel particulate filter (“DPF”) which collects the soot, followed by oxidation of the accumulated particulates at elevated temperatures which regenerates the filter.
The first types of diesel particulate filters were installed in urban buses and passenger cars as early as 1986. However, a problem with the use of such diesel particulate filters is that regeneration of the filter by soot oxidation is difficult due to the low temperatures of diesel exhaust gases (e.g., less than about 200° C.), which are unfavorable for soot oxidation. Accordingly, it is necessary to use periodic high-temperature regeneration of the filter to oxidize the soot at elevated temperatures.
More recently, a particulate filter system has been used in European diesel passenger cars which relies on fuel-borne catalytic assistance in the regeneration of the DPF, i.e., the inclusion of a catalyst, typically metal, in the fuel as an additive which functions to lower the temperature at which carbon combusts. However, such a system is complex and requires additional components such as a tank for fuel additives, an additive dosing system, and infrastructure to refill the additive fuel tank. In addition, the use of fuel-borne catalysts can lead to the formation of ash which accumulates on the filter, causing gradual loss of filter soot capacity and a decrease in time between regeneration events. Therefore, it is necessary to change the filter after about every 80 K kilometers.
Also known in the art are “continuously regenerating traps” (CRT®), which comprise a platinum-based diesel oxidation catalyst (“DOC”) positioned upstream of a diesel particulate filter. Such a trap uses NO2 generated on the Pt-containing DOC for soot oxidation. However, when used in diesel passenger cars, the amount of NOx emitted is insufficient to provide complete soot oxidation due to lower NOx/soot ratio. Accordingly, it is still necessary to use periodic high-temperature regeneration of the filter using oxygen from air to oxidize the soot at elevated temperatures.
Another known method for removing soot is to deposit a catalyst on the walls of the DPF, also referred to as a “catalyzed DPF.” A catalyzed soot filter typically comprises one or more platinum group metal catalysts and is less complex than fuel-borne catalysts. See, for example, WO 00/29726, EP 0160482, EP 0164881 and U.S. Pat. No. 5,100,632. See also WO 01/12320, which teaches the use of platinum or palladium-containing oxidation catalysts within a diesel particulate filter to improve soot oxidation on the filter.
Catalyzed DPFs containing platinum group metals are also described in DE 10214343A1, U.S. Pat. No. 4,900,517, and EP1055805. See also U.S. Pat. No. 5,746,989, EP 0758713, JP 2003278536, and EP1398069, which teach a diesel particulate NOx reduction system utilizing platinum group metals. See also US 2002-127252, EP 0658369B1, U.S. Pat. Nos. 5,330,945, 4,759,918, 5,610,117, 5,911,961, 6,143,691 and JP 11253757, which teach particulate filters or traps containing platinum group metals.
However, in the catalyzed DPF system, the contact between the catalyst and soot is relatively loose, resulting in low catalyst activity for soot oxidation. Thus, in the catalyzed DPF system, it is more difficult to oxidize soot, and the oxidation typically requires higher temperatures. Further, regeneration of the filter requires that the catalyst be able to withstand temperatures of up to 1000 to 1200° C. This is a problem for noble metal catalysts as intensive sintering of such catalysts occurs at temperatures above 750° C.
Another problem with catalyzed diesel particulate filters currently in use is that they typically employ platinum group metal (PGM) compositions, particularly platinum-based formulations which are provided in the form of a catalytic coating, or washcoat. Such a coating is very expensive to manufacture due to the high cost of platinum group metals. Accordingly, it would be desirable to be able to eliminate all or part of the platinum group metals used in diesel particulate filters.
In addition, catalyst coatings containing platinum group metals are highly active and may result in undesirable reactions such as oxidation of SO2 to SO3, and the formation of sulfated ash and sulfated particulate. In order to minimize these side effects, many catalyst suppliers have tried to decrease the concentration of platinum group metals in catalyst coatings. However, this leads to lower activities in soot oxidation, thus compromising efficiency.
In commonly-assigned (Ford) application EP 1356864, a catalytic composition is taught for soot oxidation which is free of platinum group metals. However, such a composition is adapted for use in soot oxidation in the presence of NOx as a soot oxidant, not to high-temperature soot oxidation in the presence of oxygen.
Accordingly, there is still a need in the art for a catalyst which can be used in a diesel particulate filter, which contains little or no platinum group metals, and which can effectively oxidize soot by oxygen during periodic high temperature regenerations.